At the beginning of all plant design processes the range of products to be painted and the capacity required must be considered. In this particular case it is an SUV with 4 model variants, 26 units per hour and 15 square meters of exterior surface to be painted. In addition there is a van with 20 different model variations, 30 units per hour and an exterior surface of 20 to 40 square meters to be painted. With this type of vehicle the interior also requires automated painting, which, as it is a van with a large cargo volume, requires a vast expanse to be painted. Let’s not forget the truck cab with two to three units per hour and a surface area needing to be painted that is about the same size as the van. It should become clear quite quickly that this enormous variety of vehicles cannot be processed on a single production line. A division of the line will be required on capacity grounds as well. How does the plant concept look?
A paint shop for an extremely wide range of products: a van and a minibus with 20 different model variations, a truck cab taking up only a small proportion of total production and an SUV.
For commercial vehicles, two different processes are intended: monocoat painting with only one top coat application as well as a complete triple coating system. Small and medium sized vehicles represent the bulk of production, the larger ones representing only about 11%.
The standard layout of a paint shop is based on a sequential painting line. The vehicles can pass through continuously or in cycles. At each station only a part of the overall painting scope is completed. In principle, there is also the possibility to incorporate separate “parallel” painting stations to form a so-called box concept. The characteristic feature of this concept is to perform the highest possible painting scope, if not the complete scope, in one station. On closer inspection, both concepts provide very specific benefits. While the traditional painting line is particularly suitable for high throughputs, “the box concept” offers a flexible solution for a wide range of products.
Line layout as compared to the box concept, characteristic features: low model variance, constant cycle time, and process-specific application argue in favor of the line. Individualized cycle time depending on vehicle size, process and painting scope, as well as flexible adaptation to capacity requirements are the advantages of a box concept.
Back to the specific task. For the van the box concept seems to be the less expensive solution. But how does the actual layout look? Here's where the painting process comes into play. A so-called 3wet process should be implemented. 3wet means nothing more than applying all three paint applications, i.e. primer, basecoat and clear coat, consecutively, without intermediate drying. The entire film is cured in a dryer only after the last paint application. This is a compact process that can have particularly advantageous effects. With that said now the plant layout can be sketched. The vans first pass through a cleaning station. After that the primer application occurs on the exterior surfaces, here likewise consolidated in a throughput station. Then they are distributed to the individual boxes. In each box the complete base coat and clear coat application is performed for both the interior and exterior. Each box can be operated with a variable cycle time adapted to the respective painting task. In this way only three boxes are required for the given capacity of 32 vehicles per hour. A fourth box for future capacity expansion can be easily integrated into the layout.
Is the most favorable solution really found with the concept described above? In order to answer this question quantitatively, a line concept is designed for the same task with the same boundary conditions. The first two stations, the cleaning and primer application, are identical to the box concept. After that there is a series of zones that operate in stop & go operation. The van is then successively painted, at first inside, then outside, each time with base coat and clear coat. A limitation quickly becomes apparent. The number of stations required depends on the largest vehicle, in this case a van with 40 square meters of exterior surface area and a share of 11% of total production. This means that the system is simply too large for the vast majority of smaller vehicles. The plant length, in terms of the paint spray separation system, is much bigger, and thus its energy consumption as well. The number of robots is higher and therefore also the loss through color change. The actual comparison confirms the assumption: the box concept provides a solution with a reduced investment and lower operating costs in this case.
There remains the question about whether the box or line concept is best for car painting. With four model variants, the variety is kept within limits. The painting scope and the number of process steps are similar for all variants. Interior painting is done manually. Recommended here as the most favorable solution is a line concept of the simplest design: with continuous flow and fixed six-axis painting robot.
Box versus line for van painting, a concrete comparison of concepts:
The total number of robots amounts to 42 devices for the box concept. In contrast, 64 robots are needed in the painting line. The length of the washout system for the boxes is significantly shorter at 58m than that of the line, which is 91m. This results in a savings in energy consumption of around 36% and about 31% in investment costs. The additional costs for more complex materials handling as well as the ventilation for the transfer zones in the box concept are considered. The reduced color change losses with the box concept should be mentioned. This amounts to nearly 5 tons of paint per year.
A modular box concept also brings advantages even when plants are to be designed for low quantities. Interior and exterior painting can be combined into one zone in this case. For top coat processes with waterborne base coats and clear coats based on organic solvents, two consecutive stations separated by an intermediate dryer are sufficient. Given sufficient cycle times, the entire top coat painting can be done with just two painting robots per zone. Even here the advantages are obvious: reducing by half the painting zones, the number of robots, the energy costs, and the color change losses. The prerequisite for this is a compact and powerful atomizer that allows a variable spray pattern in a wide range and likewise can be used for both interior and exterior painting. Such innovative painting concepts have already gone into use recently. These may be viewed at renowned manufacturers of premium cars – in Germany, not in China.
Flexible automation in a modular painting booth. First all interior surfaces such as door sills, engine and trunk compartments are painted. A robot with a long reach, a so-called swing arm robot, is required for the trunk compartment.
While the swing arm robot is still painting in the trunk compartment, the van's exterior painting is already being performed. In a 3-layer paint build-up the second metallic paint application comes next, followed by a clear coat layer inside and out. This all takes place in one and the same box and is only possible through a flexible application technology.
Back to the originally presented problem: designing a highly flexible and efficient paint shop for an extremely wide range of products. The optimal solution is two different layouts united under one roof; using a box concept for commercial vehicles with a high number of different variants, and a line concept for passenger cars with a relatively small range of models. Both approaches complement each other ideally.
Sources:
- Schumacher, H.: Modular Paint Box Concepts („Modshops“) in Comparison to Conventional Sequential Paint Lines. SURCAR, June/July 2011.
- Schumacher, H., Svejda, P.: Process Chain “Painted Car Body”. Strategies in Car Body Engineering 2011. Bad Nauheim, March 2011.
- Svejda, P.: Mehr Leistung bei geringerer Komplexität. JOT 3, 2010.
